A type of dry vacuum pumps comprise a pair of rotors having lobes shaped in an involute or cycloid curvature. These rotors rotate inside a pump casing while closely coupled with the lobes to each other, thereby to pump fluid. Between the lobes, there is a clearance of about 100 .mu.m so as to avoid using a lubricant.
Many conventional rotors have been made from a solid block of material. There are various way of making such solid rotors, including a method of casting solid lobes and a shaft integrally; or casting solid lobes with an inserted solid shaft; or attaching lobes to a shaft with a key. However, such solid rotors are heavy and are difficult to handle during fabrication and assembly operations. Furthermore, they are loaded with a large inertial moment, and a large torque is required for their dynamic control. For these reasons, construction of a hollow rotor can be considered, where the rotors may be fabricated from sheet or plate materials.
In such pumps, since the hollow rotor is subjected to pressure change while the pump is operated, it is necessary to provide a vent hole in the rotor so that it is not deformed due to the pressure difference between the inside and outside of the rotor. However, in this case, there is a potential danger contaminating rusting of the interior of the rotor due to the effects of humidity in storage or corrosive process gases. Also. There may be a danger to humans from residual gases trapped inside the rotor which may be released during maintenance and repairs of the pump.